1. Field of the Invention
The present invention relates an apparatus for use in impregnating fiber reinforcement with a resin for use in filament winding operations, to a process for the preparation of resin impregnated fiber reinforcement using the apparatus, and to filament winding employing resin impregnated reinforcing fibers produced thereby.
2. Description of the Related Art
Filament winding processes for preparing fiber-reinforced articles are now well established. Typically in such processes, resin impregnated strands, yarn, fabric, or tape of high-strength reinforcement fibers are used when making filament wound articles. Traditionally, these fibers are impregnated using thermosetting resins such as unsaturated polyester, epoxy, vinyl ester, polyurea, isocyanurate, and polyurethane resins. The reinforcement fibers are impregnated via passing them through either a resin bath, over an applicator roller, or of recent design, using an injection die.
Typically, when impregnating reinforcement fibers using a resin bath, the fibers are drawn through a large open bath of premixed resin. While in the bath the fibers are “weaved” back and forth through a series of bars, so as to press the resin through the fibers impregnating them. After passing through the resin bath, the fibers are then drawn through a bushing, sandwiching the fibers within the opening and squeezing out the excess resin. U.S. Pat. No. 5,766,357 describes a typical device used to sandwich the fibers as consisting of multiple sets of brass half rings and rubber plugs. Setup of the system can be difficult and time consuming, especially as the size of the bath and the number of bars through which the fibers are weaved increases. Generally speaking, the bath is a messy operation and requires an extreme excess of resin to insure saturation of the fibers, resulting in significant resin waste during processing and clean-up. The process also involves many resin-wetted parts, which increases the time required for clean-up, break-down and set-up of the system. These systems are also limited to employing resins with long gel times so as to maintain a sufficiently low viscosity within the bath to consistently wet-out the fibers, and especially, to avoid gelling the large volume of resin within the bath. Running time is often limited due to gelation of resin in stagnant areas of the resin bath, requiring shut down, removal of resin, clean up, and restart.
As fiber speeds through the bath increase, so to does the tension applied to the fibers and the finished part. This tension increases the wearing of the glass and negatively affects the strength of the finished part. Also, a “churning” of the resin occurs in the bath which entraps air within the resin and thus on the wetted fibers as they pass through the bath. This air is then entrapped within the filament wound part causing voids within the laminate, and thus weakening the part. The fibers experience additional wear and tension when sandwiched for “squeezing” to remove excess resin and provide full impregnation. During squeezing the brass and rubber edges of the bushings exert force and tension on the fibers in order to squeeze out the excess resin and as a result the fiber is put under increasing stress that causes the fiber to break and “fray” which weakens the part, creates the need to clean the applicator and tooling more frequently, and worsens the aesthetics of the finished composite part or article. The tension and its effects worsen when either of the edges used to squeeze the fibers is static (i.e. non-rotating). The disclosure of U.S. Pat. No. 5,766,357 and the references cited therein are herein incorporated by reference.
A typical applicator roller system along with some of the system's drawbacks is also described in the aforementioned U.S. Pat. No. 5,766,357. In these prior art systems, the fiber is drawn across an open wheel which dips into a reservoir of premixed resin (“kiss roll”). The system uses a knife blade or doctor blade to control the thickness of the mixed resin layer adhering to the roller. The fiber is impregnated by rolling it across the roller and through the adhering resin layer. Setup of the system can be difficult and time consuming especially when a roller and doctor blade are used for resin impregnation. In these systems, controlling the thickness of the adhering resin layer is very difficult. The viscosity of resin, which changes over time, and temperature and humidity all affect the adhering resin layer. Also, the speed and tension with which the fiber is drawn changes the impregnation roller speed which in turn affects the hydraulic pressure of the resin between the doctor blade and the resin impregnation roller. Finally, the doctor blade is set while the system is stopped and cannot generally be adjusted while the fiber is being drawn over the impregnation roller. Therefore, in systems like this, controlling the doctor blade is very difficult, but is critical to ensuring the proper amount of resin is incorporated into the fiber. A further and important drawback is that a large areal surface of resin is exposed to the air on the rollers, which will cause advancement of certain of the resins, particularly those which are moisture sensitive.
Under prior known systems, the fiber tension and speed have been found to have a profound influence on the amount of resin incorporated into the fibers. Fiber tension and speed can change in a system where the fiber is moved across an open wheel. Since the resin is incorporated through capillarity, the amount of resin pickup may vary considerably. Also, since the fiber passes over a coated wheel, only the side in contact with the wheel contacts the resin. This may lead to non-uniform impregnation and poor incorporation into the final manufactured part. Finally, the resin used in the manufacture of certain articles ages constantly and quickly changes its viscosity, especially in situations where the resin is held in an open reservoir at room temperature. A change in the viscosity of the resin also affects the amount of resin entering the fiber by capillarity. To reduce this problem the resin is generally replaced every four to eight hours. Replacement of the resin is wasteful and further requires disposal of unused hazardous materials, which increases manufacturing costs significantly.
U.S. Pat. Nos. 5,766,357 and 6,179,945 disclose an improvement in filament winding wherein the reinforcement material passes through a die or manifold into which matrix resin is injected. As a result, impregnation baths and potentially applicator rollers are eliminated. However, such injection or manifold applicators tend to apply increasing tension and cause fiber wear at high speeds due to the friction caused by squeezing the fibers through a die or manifold in which the contact edges are static (i.e. non-rotating). As previously stated, when the fiber is put under increasing stress the fiber will break and “fray” which weakens the part, creates the need to clean the applicator and tooling more frequently, and worsens the aesthetics of the finished composite part or article. Moreover, differently sized fiber materials require a distinct die adapted in geometry to coincide with the shape of the fiber reinforcement being applied. Thus, different sizes of yarn, tow, etc. will require a different die or manifold, as will tape opposed to yarn, etc. U.S. Pat. No. 6,387,179 also discloses a die type device. The disclosure of U.S. Pat. Nos. 6,179,945 and 6,387,179 and the references cited therein are herein incorporated by reference.
Thus, to date, balancing consistent wetting and full impregnation while delivering increased line speeds with minimal air entrapment, tension, and detriment to fibers caused by the fraying of these fibers has proven difficult to achieve. Furthermore, balancing the resin to glass ratio so as to promote maximum impregnation, while leaving minimal excess resin to be lost during the processing of the reinforcement fibers has also proven difficult using these traditional impregnation means (i.e. baths, applicator rollers, etc.). And, as stated in U.S. Pat. No. 6,179,945 there is a need for an improved filament winding impregnation process and apparatus whereby: 1) higher application rates of fiber reinforcement material can be wetted to reduce the time required to form a filament wound part; 2) filament wound parts can be formed with a higher reinforcement content; 3) voids in final parts or articles can be reduced; and 4) improved resin utilization occurs.